Ever since the 2009 climate talks in Copenhagen, world leaders have agreed on 2 degrees Celsius (3.6 degrees F) as the maximum acceptable global warming above pre-industrial levels to avert the worst impacts of climate change (today we’re at about 0.8 degrees C). But a new study, led by climatologist James Hansen of Columbia University, argues that pollution plans aimed at that target would still result in “disastrous consequences,” from rampant sea-level rise to widespread extinction.

A major goal of climate scientists since Copenhagen has been to convert the 2 degree limit into something useful for policymakers, namely, a specific total amount of carbon we can “afford” to dump into the atmosphere, mostly from burning fossil fuels in power plants (this is known as a carbon budget). This fall, the U.N.’s Intergovernmental Panel on Climate Change pegged the number at 1 trillion metric tons of carbon, or about…